The biological characteristics of H-2 antigens solubilized by papain digestion and chromatography continue to be investigated as to their capacity to immunize, enhance and tolerize. Several antigens are being compared: H-2a, H-2b and H-2d and various biochemical approaches to solubilization and purification are being attempted. Purification (75%-80%) has been achieved using DEAE, lectin column and G100 superfine Sephadex chromatography of the H-2a antigen. All alloantigenic determinants are preserved in this procedure and biologic assays are in pogress. Work continues with the biologic characterization of the membrane antigens of VSTA type (SV-40 induced and MuLV-induced). Several in vitro and in vivo assays of the solubilized products of these antigens continue to be studied with the hope that both H-2 and VSTA will eventually lead to a complete biochemical characterization. Solubilization and purification of both types of VSTA through G-150 Sephadex chromatography preserves the capacity to immunize syngeneic mice against their tumors. Positive reactions, using soluble SV40-induced antigen has been obtained in lymphocyte stimulation assays.